In addition to its role as a transient carrier of genetic information within a cell, RNA is now known to play a variety of functional roles in several biological processes including tRNA processing, intron splicing, and peptide-bond formation during translation (Doudna, J. A., and Cech, T. R. (2002). The chemical repertoire of natural ribozymes. Nature 418, 222-228; Moore, P. B., and Steitz, T. A. (2002). The involvement of RNA in ribosome function. Nature 418, 229-235). The recent discovery of a class of small RNAs that block translation by base pairing to the 3′-untranslated region of mRNAs reveals that natural RNAs can also regulate gene expression (Lau, N. C., Lim, L. P., Weinstein, E. G., and Bartel, D. P. (2001). An abundant class of tiny RNAs with probable regulatory roles in Caenorhabditis elegans. Science 294, 858-862). O'Malley and co-workers recently discovered an RNA that plays a structural role in a protein-RNA complex that co-activates genes regulated by steroid hormone receptors (Lanz, R. B., McKenna, N. J., Onate, S. A., Albrecht, U., Wong, J., Tsai, S. Y., Tsai, M. J., and O'Malley, B. W. (1999). A steroid receptor coactivator, SRA, functions as an RNA and is present in an SRC-1 complex. Cell 97, 17-27; Lanz, R. B., Razani, B., Goldberg, A. D., and O'Malley, B. W. (2002). Distinct RNA motifs are important for coactivation of steroid hormone receptors by steroid receptor RNA activator (SRA). Proc Natl Acad Sci USA 99, 16081-16086). An RNA molecule that functions as a transcriptional activation domain, however, has not yet been discovered in nature.
The activation of transcription in the nucleus of a cells requires multiple interactions of proteins and DNA at the start site of the gene. Ptashne and co-workers have shown that DNA-bound protein activators are required to recruit other transcription factors to the start site to begin transcription (Ptashne, Nature 335:983, 1988). In eukaryotes, for example, activation of RNA polymerase II genes requires many transcription factors in addition to RNA polymerase. Transcriptional activators have been shown to contact one or another of these transcription factors including the TATA-binding protein (TBP), TBP-associated factors (TAFs), TFIIB, and TFIIH (Roeder, Trends Biochem. Sci. 16:402, 1991; Zawel et al., Prog. Nucl. Acids Res. Mol. Biol. 44:67, 1993; Conaway et al., Annu. Rev. Biochem. 62:161, 1993; Hoey et al., Cell 72:247). Therefore, the initiation of transcription involves a multistep assembly process of which activators play a key role. (Buratowski et al. Cell 56:549, 1989; Choy et al., Nature 366:531, 1993). Some transcriptional activators are thought to recruit transcriptional factors to the DNA while others are thought to cause conformational changes in target proteins thereby facilitating the assembly of the transcriptional machinery (Lin et al. Cell 64:971, 1991; Roberts et al. Nature 371:717, 1994; Hori et al., Curr. Opin. Genet. Dev. 4:236, 1994).
Transcriptional activation has been studied both in the context of controlling gene expression in cells, for example so that the principles of gene activation might be employed in genetic therapies, and as an experimental tool for analyzing protein-protein interactions in the cell (Fields et al. Nature 340:245, 1989; Gyuris et al., Cell 75:791, 1993). There remains a need for novel transcriptional activators, especially ones that are able to be switched on and off.